1. Field of the Invention
The present invention relates to a passive seat belt system, and more particularly to a passive seat belt system having means for securing safety of a vehicle occupant when detection means for controlling actuation of the seat belt has a fault.
2. Related Background Art
A passive seat belt system in which a slidable anchor fixed to a seat belt is moved forward and backward of a car body as a door is closed and opened to fasten and release a vehicle occupant is known by, for example, U.S. Pat. Nos. 4,410,061 and 4,321,979. The former has a control circuit which uses a timer and the latter has an alarm device.
FIG. 1 shows an example of a known timer-controlled passive seat belt system. Numeral 1 denotes a door switch which is opened when a door is open, numeral 2 denotes a forward timer which monitors a movement time of the seat belt in the direction toward a forward end of a slider (not shown), numeral 3 denotes a backward timer which monitors a movement time of the seat belt in the direction of a backward end of the slider, numeral 4 denotes a forward end switch which is opened when the slider reaches the forward end, and numeral 5 denotes a backward end switch which is opened when the slider reaches the backward end.
If the seat belt is in a fasten state and the door is opened to open the door switch 1, the timer 3 is reset and an input terminal of the timer 2 is rendered HIGH so that the timer 2 is activated to turn on a transistor 7 which energizes a relay 9A to drive a motor M until the slider reaches the forward end and the forward end switch is opened.
When the seat belt is in a release state and the door is closed to close the door switch 1, an input terminal of an inverter 6 is grounded and an output of the inverter is rendered HIGH so that the timer 2 is reset and the timer 3 is activated to turn on a transistor 8 which energizes a relay 9B to drive the motor M in the opposite direction until the slider reaches the backward end and the backward end switch 5 is opened.
In the prior art passive seat belt system, if the door switch has a fault such that the input terminal thereof is opened, it is always determined as a door open state, and the slider does not move backward so that the seat belt does not assume the fasten state. On the other hand, if the door switch is shorted, it is always determined as a door close state, and the slider does not move forward so that the seat belt does not assume the release state. In the former case, the vehicle occupant is not fastened and the seat belt does not function as intended, thereby causing poor safety.